In recent years, oil wells are being deeper and are being developed in increasingly severe environments. For this reason, there is a strong desire that tubular threaded joints which are used to connect oil country tubular goods including tubing and casing for oil or gas wells have increased resistance to compression under internal and external pressures and improved sealing properties.
FIG. 1A is a cross-sectional view schematically showing the structure of a typical threaded joint for pipes, FIG. 1B is an enlarged schematic cross-sectional view of portion A in FIG. 1A, and FIG. 1C is a partial schematic cross-sectional view of the vicinity of the lip portion of a threaded joint for pipes disclosed in Patent Document 1.
As shown in FIGS. 1A-1C, the tubular threaded joint 0 is constituted by a pin 1 formed on the outer surface of both ends of steel pipes and a box 2 formed on the inner surface of a coupling which is a separate member. The pin 1 has a threaded portion 3 having male threads (external threads) and a lip portion 4 which is a portion closer to the pipe end. The lip portion 4 has a sealing surface 5 adjacent to the threaded portion 3 and a shoulder surface 9 on the front end surface of the pin 1. Correspondingly, the box 2 has a threaded portion 7 having female threads (internal threads), a sealing surface 8, and a shoulder surface 11. The threaded portions, sealing surfaces, and shoulder surfaces of the pin and the box constitute contact surfaces of a threaded joint.
A threaded joint is designed such that when the male threads and the female threads are tightened until the shoulder surfaces 9 and 11 of the pin and the box contact each other with a predetermined torque, their sealing surfaces 5 and 8 intimately contact each other with a predetermined interference to form a metal-to-metal seal which guarantees the desired gas tightness of the threaded joint. Prior to tightening, lubricating grease (typically compound grease) has conventionally been applied to the contact surfaces of the threaded joint in order to prevent the occurrence of galling of the threaded joint.
In the threaded joint for pipes shown in FIG. 1C, the lip portion 4 of the pin 1 extends in the axial direction to form a non-contacting region 13 where the pin and the box do not contact each other between the sealing surfaces 5, 8 and the shoulder surfaces 9, 11 of the pin and the box. By extending the lip portion in this manner, the pin has an increased resistance to deformation of the portion which is closer to its front end than the sealing surface. As a result, it is difficult for the sealing surface of the pin to deform even under a combined load of pressure and axial force, thereby making it possible to improve the gas tightness of the threaded joint.
The illustrated threaded joint has another non-contacting region 14 between one or more male threads of the pin 1 closest to the sealing surface 5 and the opposing surface of the box 2. The non-contacting region 14 is formed by providing a recess in the surface of the box and functions as a reservoir to collect lubricating grease expelled from the threaded portions which are tightened during makeup of the threaded joint.
In the threaded joint shown in FIG. 1C, the pin 1 has a second shoulder surface 10 between the shoulder surface 9 and the non-contact surface 13. The second shoulder surface 10 has a larger angle of slope with respect to a plane perpendicular to the pipe axis and a smaller radial dimension compared to the shoulder surface 9. The inner, larger shoulder surface 9 is referred to as a main shoulder surface, and the outer, smaller shoulder surface 10 is referred to as a sub-shoulder surface. Correspondingly, the box 2 has a sub-shoulder surface 12 in addition to the shoulder surface 11, which is a main shoulder surface. The main shoulder surfaces 9 and 11 of the pin and the box serve to withstand the compression stress applied during makeup of the threaded joint and also limit the radially inward deformation of the end of the lip 4, while the sub-shoulder surfaces 10 and 12 thereof serve to limit the radially outward deformation of the main shoulder surfaces when the main shoulder surfaces receive the compression stress. As a result, the main shoulder surfaces of the pin and the box can abut in a stable manner.
When the scaling surfaces and the shoulder surfaces of the pin and the box of a threaded joint come into intimate contact with the opposing surfaces by makeup of the threaded joint, the non-contacting region 13 which is located between the sealing surfaces and the shoulder surfaces becomes a closed space. The lubricating grease and the product fluid which are expelled from the intimately contacted sealing surfaces and shoulder surfaces flow into the closed space of the non-contacting region 13 and are confined therein. If the pressure of the fluid confined in the non-contacting region 13 becomes high due to an increased amount of the fluid, the non-contacting region 13 tends to radially expand due to the pressure, and there is the possibility of a deterioration in the gas tightness of the threaded joint which is achieved by the intimate contact between the sealing surfaces of the pin and the box.
Therefore, the threaded joint disclosed in Patent Document 1 has at least one groove which has a depth of at least 0.1 mm in the shoulder surface of at least one of the pin and the box and which functions as a leak path for high pressure fluid confined in the non-contacting region 13.
FIGS. 2A-2D are explanatory views showing grooves formed in a shoulder surface of a pin. As shown in this figure, groove portions 9a-1 and 9a-2 (which cooperatively form grooves 9a) are formed in the sub-shoulder surface 10 and the main shoulder surface 9, respectively, of a pin 1.
The grooves 9a run across both the main shoulder surface 9 and the sub-shoulder surface 10 of the pin 1. The grooves 9a may be formed in the shoulder surface of a box, or portion of the grooves 9a may be formed in a shoulder surface of the pin 1 with the remainder being formed in a shoulder surface of the box 2. The grooves 9a connect the non-contacting region 13 with the interior of the tubular threaded joint 0. Therefore, even if the fluid confined in the non-contacting region 13 produces a high pressure, the high pressure fluid can escape to the interior of the tubular threaded joint 0 through the grooves 9a, and the state of contact between the sealing surfaces 5 and 8 does not change, thereby maintaining the gas tightness of the threaded joint.
When performing makeup of a tubular threaded joint, a liquid lubricating grease containing a large amount of heavy metals has conventionally been applied each time makeup is carried out. From the standpoints of environment protection and working efficiency, tubular threaded joints having a surface coated with a solid lubricating coating which does not discharge pollutants such as heavy metals to the surroundings have been developed.
FIG. 3 is an explanatory view showing the coating structure formed on the surfaces of a tubular threaded joint disclosed in Patent Document 2, which is an example of a tubular threaded joint having such a solid lubricating coating. In a tubular threaded joint 15 which is constituted by a pin 1 and a box 2, the contact surface of the pin 1 has a preparatory surface treatment coating 18 which may optionally be provided on a steel substrate 17 for the purpose of surface roughening, and atop it a solid anticorrosive coating 19 based on a UV curable resin. The contact surface of the box 2 has a preparatory surface treatment coating 21 which may optionally be provided on the steel substrate 20 for the purpose of surface roughening and atop it a solid lubricating coating 22.
The solid lubricating coating 22 is a coating exhibiting plastic or viscoplastic rheological behavior in which the fluidity of the coating markedly varies with pressure. A coating having such properties can exhibit a higher galling resistance compared to a solid lubricating coating which does not have the above-described rheological behavior (such as a rigid coating made of a thermosetting resin containing a lubricating powder). In addition, this type of coating can exhibit a self-repairing function due to its fluidity, which is increased under pressure.